North American Lauraceae: Terpenoid Emissions, Relative Attraction and Boring Preferences of Redbay Ambrosia Beetle, Xyleborus glabratus (Coleoptera: Curculionidae: Scolytinae)

Abstract

The invasive redbay ambrosia beetle, Xyleborus glabratus, is the primary vector of Raffaelea lauricola, a symbiotic fungus and the etiologic agent of laurel wilt. This lethal disease has caused severe mortality of redbay (Persea borbonia) and swampbay (P. palustris) trees in the southeastern USA, threatens avocado (P. americana) production in Florida, and has potential to impact additional New World species. To date, all North American hosts of X. glabratus and suscepts of laurel wilt are members of the family Lauraceae. This comparative study combined field tests and laboratory bioassays to evaluate attraction and boring preferences of female X. glabratus using freshly-cut bolts from nine species of Lauraceae: avocado (one cultivar of each botanical race), redbay, swampbay, silkbay (Persea humilis), California bay laurel (Umbellularia californica), sassafras (Sassafras albidum), northern spicebush (Lindera benzoin), camphor tree (Cinnamomum camphora), and lancewood (Nectandra coriacea). In addition, volatile collections and gas chromatography-mass spectroscopy (GC-MS) were conducted to quantify terpenoid emissions from test bolts, and electroantennography (EAG) was performed to measure olfactory responses of X. glabratus to terpenoids identified by GC-MS. Significant differences were observed among treatments in both field and laboratory tests. Silkbay and camphor tree attracted the highest numbers of the beetle in the field, and lancewood and spicebush the lowest, whereas boring activity was greatest on silkbay, bay laurel, swampbay, and redbay, and lowest on lancewood, spicebush, and camphor tree. The Guatemalan cultivar of avocado was more attractive than those of the other races, but boring response among the three was equivalent. The results suggest that camphor tree may contain a chemical deterrent to boring, and that different cues are associated with host location and host acceptance. Emissions of α-cubebene, α-copaene, α-humulene, and calamenene were positively correlated with attraction, and EAG analyses confirmed chemoreception of terpenoids by antennal receptors of X. glabratus.

Laurel Wilt in Natural and Agricultural Ecosystems: Understanding the Drivers and Scales of Complex Pathosystems

“…The strongest female attractants identified to date are terpenoid kairomones, specifically volatile sesquiterpenes emitted from host wood [80][81][82]. In comparative studies with nine lauraceous species, emissions of four sesquiterpenes (α-copaene, α-cubebene, α-humulene, and calamenene) were positively correlated with in-flight attraction of X. glabratus, and electroantennography has confirmed olfactory chemoreception of these compounds [66]. A succession of field lures has been developed using essential oils naturally high in sesquiterpenes, including manuka oil (derived from Leptospermum scoparium J.R. Forst.…”

“…Thus, X. glabratus is unusual as it displays a strong preference for trees in the Lauraceae. Sesquiterpenes, rather than ethanol, the stress metabolite to which ambrosia beetles are usually attracted, appear to be a significant component of the attraction signature of these trees [66]. This difference has been cited when indicating that the X. glabratus vector relationship is exceptional, but there appears to be no evidence that trees identified with these signatures then support colonization and brood development before disease develops.…”

“…To minimize risks associated with dispersion, it would be adaptive for females to have efficient host-seeking behaviors, guided by reliable cues. Kendra et al [66] proposed that host location and acceptance is a multistep process directed by a series of cues presented in sequential order.…”

“…With X. glabratus, females engage in host-seeking flight during the late afternoon and early evening, several hours earlier than other species of Xyleborus in Florida [90]. While in flight, females orient initially toward long-range olfactory cues; α-copaene appears to be the primary kairomone, but other terpenoids likely contribute to generate an attractive "signature bouquet" of the Lauraceae [66,82,87]. In addition to sesquiterpenes, several monoterpenes have been reported as kairomones, including eucalyptol (1,8 cineole) [91] and a blend of redbay leaf volatiles [92].…”

“…[26], their attractiveness to X. glabratus [66,78], the rapid increase in X. glabratus numbers in affected stands of these trees [58], and the substantial amounts of inoculum that most females of X. glabratus carry [70]. Furthermore, tree-to-tree spread via interconnected root systems, which has been suspected in avocado [28] and sassafras [147], and demonstrated in pondberry [148], enables movement of the pathogen without vector assistance.…”